If Pokémon leveled up slower, the games would feel like a grind. If you halved the experience your Pokémon would receive, you’d likely not have a Wartortle until your fourth or fifth gym, and your precious Blastoise wouldn’t be evolved until the elite four or sometime after. Most gamers can get their starter into its second stage by the first or second gym easily. The gym leaders would also have to be nerfed so their Pokémon would be at lower levels. The canonical problems of this story-wise are pretty obvious. This method would kill any fun in both the story and in the actual gameplay. It would also make it impossible for competitive gamers to even begin working towards their teams until they’ve played the equivalent of two or three full Pokémon games in playtime.
Lastly is character customization. Again, this is a feature that we got in Pokémon X and Y, but it was extremely limited (especially if you had a male trainer) and we did not get it back in Alpha and Omega. I had a lot of fun visiting towns to see what clothing they had for sale, knowing they rotated styles. I had to make a point to turn on my game every day and make the rounds to complete my wardrobe. Finally, I had a trainer that felt (mostly) like my own. I know I’m not alone when I say that I have a handful of trainer OC’s (“Original Characters” for those out of the loop), and when I finally could make my avatar reflect them, I was beside myself. Of course, you could never remove the darn hats and boys especially were limited in hairstyle, but this was a step in the right direction. I would personally love to see this feature make a return and expanded further. Let us create our trainers, Nintendo! If we can do it on Animal Crossing and Harvest Moon, it’s really not much to ask here!